Gas spring

ABSTRACT

A gas spring ( 7 ) comprises a cylinder main body ( 20 ); a rod member ( 21 ) slidable to cylinder main body; a gas filled chamber ( 23 ) filled with compressed gas and urging the rod member in a first direction; a rod inserting hole ( 32 ) inserting the rod member ( 21 ); an annular collar ( 24 ) slidable on the rod member at a position apart from the rod inserting hole in the second direction opposite to the first direction; an annular groove ( 24   a ) on the annular collar which is open toward the second direction; and a lock ring ( 26 ) engaging with an annular ring mounting groove ( 25 ) on the rod member ( 21 ). In assembling, the lock ring ( 26 ) is moved in the first direction together with the rod member, to thus engage with the annular groove ( 24   a ) on the annular collar, so that the annular collar for stopping the rod member and the lock ring is stopped by an end wall of the cylinder main body.

TECHNICAL FIELD

[0001] The present invention relates to a gas spring and its gas fillingmethod, more particularly, to an improved mechanism for restricting theadvancing motion of a rod member.

BACKGROUND OF THE INVENTION

[0002] A conventional gas spring for use in various machines is providedwith a cylinder main body, a rod member slidably fitted to the cylindermain body, a gas filled chamber inside of the cylinder main body andfilled with compressed gas (i.e. nitrogen gas). Compressed gas (pressureis about 7 to 10 MPa), is filled in the gas filled chamber. The rodmember is driven to advance by the compressed gas. Here, the gas springis provided with various mechanisms for restricting the advancingoperation of the rod member in such a manner that the rod member cannotadvance in excess of a predetermined stroke.

[0003] For example, the applicant of the present invention has proposeda gas spring as shown in FIG. 10. The gas spring 100 comprises acylinder main body 101; a rod member 102 with a cylindrical pistonpenetrating the cylinder main body 101; a gas filled chamber 103, whichis formed between the cylinder main body 101 and the rod member 102 andurges the rod member 102 upward with pressure of compressed gas filledinside; a plurality of pin members 104 disposed symmetrically with eachother around an axial center of the rod member 102; and slots 105 whichreceive pin members 104, so as to restrict the moving range of the rodmember 102 with respect to the cylinder main body 101. When the rodmember 102 advances upward by a predetermined stroke, each pin member104 fits to the upper end of the slot 105, thereby restricting themovement of the rod member 102.

[0004] In the above-described gas spring, after filling of thecompressed gas into the gas filled chamber 103, the rod member 102 ispressed down with respect to the cylinder main body 101, to thus turnthe gas filled chamber 103 into a sealed state. Thereafter, it isnecessary to securely screw the pin members 104 to a screw hole formedat the rod member 102 in order to restrict the movement of the rodmember 102. As a result, the man-hour of a gas filling work isincreased. Moreover, there is a possibility that the structure of thegas spring becomes complicated.

[0005] An object of the present invention is to constitute a gas springsuch that a rod member can be restricted at an upper limit position atthe same time when compressed gas is filled into a gas filled chamber,and to simplify a mechanism for restricting the advancing motion of therod member.

DISCLOSURE OF THE INVENTION

[0006] A gas spring according to the present invention is provided witha cylinder main body, a rod member slidably inserted into the cylindermain body, and a gas filled chamber inside of the cylinder main body andfilled with compressed gas and applying gas pressure on the rod memberso as to urge the rod member in a first direction.

[0007] This gas spring further comprises; a rod inserting hole forinserting the rod member which is formed on an end wall of the cylindermain body in a second direction opposite to the first direction; anannular collar slidably fitted around the rod member at a position apartfrom the rod inserting hole in the second direction, an annular grooveformed on the annular collar and being open toward the second direction;and a lock ring engaging with an annular ring mounting groove formed onthe rod member; wherein the lock ring is moved in the first directiontogether with the rod member, to thus engage with the annular groove onthe annular collar, so that the annular collar for stopping the rodmember and the lock ring is stopped by an end wall of the cylinder mainbody.

[0008] When the rod member is moved in the first direction by gaspressure of the compressed gas in the gas filled chamber in the state inwhich the lock ring engages with the ring mounting groove of the rodmember, the lock ring is moved toward the first direction together withthe rod member. When the rod member is moved by a predetermine stroketoward the first direction, the lock ring engages with the annulargroove on the annular collar, which receives the rod member and the lockring and moves toward the first direction. The annular collar is stoppedby the end wall of the cylinder main body, thereby restricting themovement of the rod member toward the first direction. Thus, the rodmember can be restricted at an upper limit position without any specialwork after filling of the compressed gas, thus simplifying thecompressed gas filling work. Furthermore, only the annular collar fittedaround the rod member and the lock ring can restrict the movement of therod member, thereby simplifying the structure of the gas spring.

[0009] Here, it is preferable that the lock ring should be constitutedsuch that its diameter can be reduced via one separated portion. Whenthe lock ring is located apart from the ring mounting groove, thediameter of the lock ring is enlarged, so that the lock ring is fittedaround the rod member. In contrast, when the lock ring reaches theposition of the ring mounting groove, the diameter of the lock ring isreduced, so that the lock ring securely engages with and is held in thering mounting groove. Thus, the lock ring cannot disengage from the ringmounting groove.

[0010] Furthermore, a gas filling method according to the presentinvention for filling compressed gas into above described gas spring,comprises the steps of; filling the compressed gas into the gas filledchamber with the use of a gas filling jig; moving the rod member towardthe second direction so as to allow the lock ring to engage with thering mounting groove after filling of the compressed gas; moving the rodmember in the first direction by the compressed gas in a state in whichthe lock ring engages with the ring mounting groove, so as to introducethe lock ring into the annular groove formed on the annular collar; andlocking the annular collar on an end wall of the cylinder main body.

[0011] When the compressed gas is filled into the gas spring, the gasspring is disposed in the gas filling jig. After the compressed gas isfilled into the gas filled chamber in the state in which the lock ringdisengages from the ring mounting groove, the rod member is driven inthe second direction by drive means such as a fluid pressure cylinderfrom the outside. When the rod member is moved in the second direction,the ring mounting groove reaches the position of the lock ring, so thatthe lock ring engages with the ring mounting groove. Next, when urgingforce by the drive means is released, the rod member is driven in thefirst direction by the gas pressure of the compressed gas, and then, thelock ring is moved toward the first direction together with the movementof the rod member in the state in which the lock ring engages with thering mounting groove, so that the lock ring is introduced into theannular groove formed on the annular collar. The annular collar receivesthe rod member and the lock ring, and thus, is stopped on the end wallof the cylinder main body, so that the rod member can be restricted frombeing moved in the first direction. Thus, it is possible to restrict therod member at the upper limit position without any special work afterfilling of the compressed gas, thereby simplifying the gas filling work.

[0012] Here, it is preferable that the gas filling jig is provided witha case main body, an outer cylindrical member to be fitted around aportion on an end side in the first direction of the rod member; a rodcontaining member containing a portion on an end side in the secondddirection of the rod member; and an inner cylindrical member extendingfrom an end in the first direction of the rod containing member towardthe first direction and containing therein the cylinder main body.

[0013] When the compressed gas is filled into the gas filled chamber inthe gas spring, the outer cylindrical member is fitted around theportion on the end side in the first direction of the rod member, andfurther, the portion on the end side in the second direction of the rodmember is contained in the rod containing member, so that the gas springis disposed in the gas filling jig by containing the cylinder main bodyin the inner cylindrical member. Subsequently, the outer cylindricalmember is moved toward the first direction, so that the rod member ismoved toward the first direction relatively with respect to the cylindermain body, thereby producing a clearance between the rod member and aseal member disposed in the rod inserting hole. Thereafter, a gasfilling device disposed outside is connected to a gas filling hole,through which the compressed gas can be filled into the gas filledchamber from the containing chamber via the filling hole.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a vertically cross-sectional view of a tool fixingdevice and a gas spring according to a preferred embodiment of thepresent invention.

[0015]FIG. 2 is a vertically cross-sectional view of the gas spring.

[0016]FIG. 3 is an enlarged cross-sectional view showing the vicinityportion of an annular collar (when the gas spring is in a usable state).

[0017]FIG. 4 is an end view showing a lock ring.

[0018]FIG. 5 is a vertically cross-sectional view showing the gas spring(when compressed gas is being filled).

[0019]FIG. 6 is a vertically cross-sectional view showing the gas spring(when filling of compressed gas is completed).

[0020]FIG. 7 is an enlarged cross-sectional view showing the vicinityportion of the annular collar (when compressed gas is being filled).

[0021]FIG. 8 is an enlarged cross-sectional view showing the vicinityportion of the annular collar (when compressed gas is being sealed).

[0022]FIG. 9 is a vertically cross-sectional view showing the vicinityportion of the annular collar (when filling of compressed gas iscompleted).

[0023]FIG. 10 is a vertically cross-sectional view of a gas spring inthe prior art.

PREFERRED EMBODIMENT OF THE INVENTION

[0024] A description of a preferred embodiment according to the presentinvention will be given hereinafter. In the present preferredembodiment, the present invention is applied to a tool fixing device forfixing a tool detachably at the tip of a spindle of a machine tool.

[0025] As shown in FIG. 1, a spindle 2 of a machine tool is supportedrotatably via a plurality of bearings 2 a, and a tapered tool holder 10is formed in the tip of the spindle 2 in such a manner that the diameterof the tool holder 10 is enlarged toward the tip thereof.

[0026] A frame 4 supporting the spindle 2 is provided with a tool fixingdevice 1 and a coil 9 for rotatably driving the spindle 2. The toolfixing device 1 comprises a collet 5 engageable with an engaging portion3 a formed at a base end of a tool 3 in an engageable or disengageablemanner; a draw bar 6 in association with the collet 5; a gas spring 7according to the present invention, connected to the draw bar 6, forurging the draw bar 6 toward the base end (on a tool fixing side,i.e.,upward in FIG. 1); and a fixture releasing mechanism 8 capable ofdriving the draw bar 6 toward the tip (on a tool fixture releasing side,i.e., downward in FIG. 1) against the urging force of the gas spring 7.

[0027] In the spindle 2, the tool holder 10, a housing hole 11 housingthe collet 5 therein, an inserting hole 12, into which the draw bar 6 isinserted, and another housing hole 13 housing the gas spring 7 thereinare formed in series from the tip toward the base end. At the tip of thehousing hole 11 formed is a tapered hole 11 a for elastically deforminga chuck of the collet 5 toward a diameter decreasing side. As shown inFIGS. 1, 2, the housing hole 13 consists of a rod sliding hole 13 a,into which a large-diameter portion 21 b of a rod member 21 is slidablyfitted, a cylinder housing hole 13 b which diameter is slightly smallerthan that of the rod sliding hole 13 a and in which a cylinder main body20 of the gas spring 7 is fitted, a recessed hole 13 c continuous to thecylinder housing hole 13 b, and a cylindrical hole 13 d having adiameter smaller than that of the recessed hole 13 c. These holes 13 a,13 b, 13 c and 13 d are arranged in series from the base end. Acylindrical member 14 is fitted into the recessed hole 13 c, so that thetip end of the cylinder main body 20 abuts against the base end of thecylindrical member 14.

[0028] The tool 3 is provided with the engaging portion 3 a, a taperedshaft portion 3 b and a large-diameter disk portion 3 c disposed at thetip of the tapered shaft portion 3 b. The collet 5 has an elastic chuck,which is divided at the tip portion thereof into three or four portions,and includes a shaft hole 5 a, into which the draw bar 6 is inserted,and a large-diameter hole 5 b continuous with the tip of the shaft hole5 a and has a diameter larger than that of the shaft hole 5 a. In thelarge-diameter hole 5 b fitted is a locking portion 6 a fixed at the tipof the draw bar 6.

[0029] When the draw bar 6 is moved toward the tool fixing side, thelocking portion 6 a is locked to a step 5 c at the boundary between theshaft hole 5 a and the large-diameter hole 5 b, and further, the collet5 is moved toward the base end. And then, the draw bar 6 intrudes deeplyinto the housing hole 11 while the chuck at the tip of the collet 5 isclosed by the tapered hole 11 a, and on the way, engages with theengaging portion 3 a of the tool 3. Thereafter, the engaging portion 3 ais pulled toward the base end, so that the tool 3 is fixed in the statein which the tapered shaft portion 3 b of the tool 3 abuts stronglyagainst the tool holder 10. When the draw bar 6 is moved toward thefixture releasing side, the locking portion 6 a is unlocked from thestep 5 c. Consequently, the elastic chuck at the tip of the collet 5also is moved toward the tip while it opens, so that the collet 5disengages from the engaging portion 3 a of the tool 3, therebyreleasing the tool 3. In other words, since the draw bar 6 is urgedtoward the tool fixing side via the engaging shaft portion fixed to thebase end of the draw bar 6 by the gas spring 7, the draw bar 6 can bedriven from the fixture releasing position to the tool fixing position,and further, the tool 3 can be fixed to the tool holder 10 while thedraw bar 6 is held at the tool fixing position. Incidentally, as shownin FIG. 1, the fixture releasing mechanism 8 includes a hydrauliccylinder 60 to be controlled and driven by a drive controller includinga hydraulic supply source. The hydraulic cylinder 60 drives the rodmember 21 in the gas spring 7 from an advance position to a retreatposition, thereby driving the draw bar 6 connected to the rod member 21toward a fixture releasing side.

[0030] Next, explanation will be made on the gas spring 7.

[0031] As shown in FIGS. 2, 3, the gas spring 7 comprises the cylindermain body 20 fitted into the spindle 2; the rod member 21 slidablyinserted into the cylinder main body 20; and a gas filled chamber 23,which is formed inside of the cylinder main body 20 and drives the rodmember 21 upward (i.e., a first direction) by the effect of a gaspressure on pressure receiving portions 42,43 of the rod member 21 inthe state in which compressed gas 22 is filled. In a portion lower thana rod inserting hole 32 (i.e., a second direction side) in the cylindermain body 20 and at the inner circumference of the cylindrical member 14disposed is an annular collar 24 slidably fitted around the rod member21, and further, an annular ring mounting groove 25 is formed on the rodmember 21. A lock ring 26 engages with and is held in the ring mountinggroove 25.

[0032] On end walls 20 a and 20 b at the upper and lower ends of thecylinder main body 20 formed are rod inserting holes 31 and 32, intowhich the rod member 21 is inserted, respectively. The gas filledchamber 23 is formed inside of the cylinder main body 20. The gas filledchamber 23 is filled with the compressed gas 22 (i.e., nitrogen gas) ofhigh pressure (7 to 10 MPa) with the use of a gas filling jig 50,described later. At the upper end of the gas filled chamber 23 disposedis a seal member 33 for sealing a clearance between the cylinder mainbody 20 and the rod member 21. A ring member 34 is disposed on the sideof the gas filled chamber 23 in the vicinity of the seal member 33. Agrease filled chamber 35 filled with grease is formed between the sealmember 33 and the ring member 34. In the same manner, at the lower endof the gas filled chamber 23 disposed are a seal member 36 and a ringmember 37, and further, a grease filled chamber 38 is formed between theseal member 36 and the ring member 37. Furthermore, seal members 39 and40 are disposed between the cylinder main body 20 and the cylinderhousing hole 13 b.

[0033] The rod member 21 is integrally formed with a connecting portion21 a to be connected to an output member 60 a of the hydraulic cylinder60, the large-diameter portion 21 b slidably fitted into the rod slidinghole 13 a, an intermediate-diameter portion 21 c slidably fitted intothe rod inserting hole 31, and a rod portion 21 d slidably fitted intothe rod inserting hole 32 and connected to the draw bar 6 at the lowerend thereof in series from above. When the rod member 21 is moveddownward by the hydraulic cylinder 60, the large-diameter portion 21 breaches the lower end of the rod sliding hole 13 a, and then, is lockedat a step 41 formed at the boundary between the rod sliding hole 13 aand the cylinder housing hole 13 b, and therefore, the rod member 21 canbe restricted from being moved further downward. The tapered pressurereceiving portion 42 is formed between the intermediate-diameter portion21 c and the rod portion 21 d. The pressure receiving portion 43 isformed on the way of the rod portion 21 d. The gas pressure of thecompressed gas 22 acts on the pressure receiving portions 42, 43,thereby urging the rod member 21 upward. A seal member 44 for sealing aclearance between the spindle 2 and the large-diameter portion 21 b isdisposed in the large-diameter portion 21 b.

[0034] Subsequently, a description will be given of the annular collar24 and the lock ring 26.

[0035] As shown in FIGS. 2 and 3, the annular collar 24 is slidablyfitted into the cylindrical member 14 in the state fitted around the rodportion 21 d of the rod member 21. At the lower end of the annularcollar 24 formed is an annular groove 24 a which is open toward downwardat the lower end thereof. The annular groove 24 a is formed at a lowerhalf of the annular collar 24. In the state in which the compressed gas22 is filled and sealed in the gas spring 7, the lock ring 26 engageswith and is held in the ring mounting groove 25 formed at the rodportion 21 d of the rod member 21. As shown in FIG. 4, the lock ring 26is constituted with a separated ring made of steel, and is constitutedso that the diameter can be reduced via one separated portion 26 a. Thelock ring 26 engages with and is held in the ring mounting groove 25 inthe state in which the rod member 21 is moved downward. In this state,when the rod member 21 is moved upward by the urging force of thecompressed gas 22, the lock ring 26 is moved upward together with therod member 21. When the rod member 21 is moved by a predeterminedstroke, the lock ring 26 engages with the annular groove 24 a. Theannular collar 24 is moved upward while receiving the rod member 21 andthe lock ring 26, to be stopped by the end wall 21 b of the cylindermain body 20, thereby restricting the upward movement of the rod member21.

[0036] Next, explanation will be made on the gas filling jig 50 forfilling the compressed gas 22 into the gas spring 7. As shown in FIGS. 5and 6, the gas filling jig 50 is provided with a case main body 51, acontaining chamber 52 which is formed inside of the case main body 51and contains the gas spring 7 therein, and a gas filling hole 53 a whichis formed in the case main body 51 and fills the compressed gas 22 intothe containing chamber 52. The case main body 51 comprises an outercylindrical member 53 fitted around the upper end portion of the rodmember 21, a rod containing member 54 containing the lower half of therod member 21, and an inner cylindrical member 55 extending upward fromthe upper end of the rod containing member 54 and containing thecylinder main body 20 therein. The gas filling hole 53 a is formed atthe side wall of the outer cylindrical member 53. A closing member 57 isfixed via a bolt at the upper end of the outer cylindrical member 53.The upper end of the rod member 21 abuts against the lower surface ofthe closing member 57 in the state in which the gas spring 7 is set.

[0037] In the rod containing member 54 formed are a containing hole 54 acontaining the lower end of the cylinder main body 20 therein, and a rodcontaining hole 54 b which is continuous to the lower end of thecontaining hole 54 a and contains the rod portion 21 d of the rod member21. A cylindrical sleeve member 58 is fitted into the rod containinghole 54 b. The rod portion 21 d is slidably fitted in the sleeve member58. A stepped portion 54 c is formed at the lower end of the rodcontaining hole 54 b, thereby restricting the downward movement of thesleeve member 58 while only the rod portion 21 d can be movedvertically.

[0038] The cylinder main body 20 is fitted to and contained in the innercylindrical member 55. The lower end of the outer cylindrical member 53is slidably fitted at the upper end of the inner cylindrical member 55.

[0039] Subsequently, a description will be given of the function of thegas spring 7. First, in order to fill the compressed gas 22 into the gasspring 7, the gas spring 7 is set in the gas filling jig 50. That is tosay, the lock ring 26 and the annular collar 24 are disposed at theupper end of the sleeve member 58; the rod portion 21 d of the rodmember 21 is inserted to the annular collar 24 and the lock ring 26, tobe thus disposed inside of the rod containing member 54; andsimultaneously, the cylinder main body 20 is fitted in the innercylindrical member 55. Thereafter, the outer cylindrical member 53 isfitted around the large-diameter portion 21 b of the rod member 21.

[0040] In this state, in order to fill the compressed gas 22 into thegas spring 7, the rod member 21 is moved upward relatively to thecylinder main body 20 such that the seal member 33 is fitted around therod member 21 at the position of the pressure receiving portion 42, asshown in FIG. 5; a gas filling device 56 is connected to the gas fillinghole 53 a in the state in which a clearance 59 is generated between therod member 21 and the seal member 33; and then, the compressed gas 22 isfilled into the gas filled chamber 23 from the containing chamber 52 viathe clearance 59. At this time, the ring mounting groove 25 ispositioned above the annular collar 24, as shown in FIG. 7, and thus,the lock ring 26 fitted around the rod member 21 is enlarged in diameterwithout any engagement with the ring mounting groove 25.

[0041] When the rod member 21 is pressed downward together with theouter cylindrical member 53 by the drive means such as the hydrauliccylinder after the compressed gas 22 is filled into the gas filledchamber 23, the rod member 21 is moved downward so that the seal member33 is fitted around the intermediate-diameter portion 21 c of the rodmember 21. In this state, the clearance 59 defined between the rodmember 21 and the seal member 33 is eliminated, thereby turning the gasfilled chamber 23 into a sealed state. At this time, the ring mountinggroove 25 reaches the position of the lock ring 26, as shown in FIG. 8,and thus, the diameter of the lock ring 26 is reduced to engage with thering mounting groove 25.

[0042] In this state, when the urging force by the drive means isreleased, the rod member 21 advances upward again by the gas pressure ofthe compressed gas 22, as shown in FIG. 6. However, the lock ring 26 ismoved upward integrally with the rod member 21, as shown in FIG. 9. Whenthe rod member 21 is moved upward by a predetermined stroke, the lockring 26 is introduced into the annular groove 24 a, and further, theannular collar 24 is moved upward integrally with the rod member 21, tobe thus stopped by the end wall 21 b of the lower end of the cylindermain body 20 in abutment, thereby restricting the upward movement of therod member 21. Thereafter, the gas spring 7 is detached from the gasfilling jig 50, thus leading to the completion of the filling work.

[0043] As shown in FIG. 2, in the case where the gas spring 7 filledwith the compressed gas 22 is disposed in the spindle 2 for use, the rodmember 21 is driven upward by the gas pressure of the compressed gas 22acting on the pressure receiving portions 42 and 43 of the rod member 21in the state in which the rod member 21 is not driven downward by thehydraulic cylinder 60 in the fixture releasing mechanism 8. At thistime, as shown in FIG. 3, the lock ring 26 is moved upward together withthe rod member 21 and engages with the annular groove 24 a of theannular collar 24 in the state in which the lock ring 26 engages withand is held in the ring mounting groove 25. When the lock ring 26engages with the annular groove 24 a, the annular collar 24 receives therod member 21 and the lock ring 26, and further, is moved upwardtogether with the rod member 21. When the rod member 21 is moved upwardby the predetermined stroke, the annular collar 24 abuts against the endwall 21 b of the lower end of the cylinder main body 20, therebyrestricting the upward movement of the rod member 21.

[0044] When the rod member 21 is moved downward, the rod member 21 isdriven downward against the urging force of the gas pressure of thecompressed gas 22 by the hydraulic cylinder 60, and thereafter, the lockring 26 is moved downward together with the rod member 21, therebyreleasing the engagement between the lock ring 26 and the annular groove24 a.

[0045] With the above-described gas spring 7, the lock ring 26 engageswith the annular groove 24 a of the annular collar 24, and further, theannular collar 24 is stopped by the end wall 21 b of the cylinder mainbody 20. Consequently, it is possible to securely restrict the upwardmovement of the rod member 21 via the annular collar 24. Moreover, thelock ring 26 is constituted so as to reduce its diameter via oneseparated portion 26 a, and therefore, the lock ring 26 can securelyengage with the ring mounting groove 25 without any failure.

[0046] The upward movement of the rod member 21 can be restricted by thelock ring 26 and the annular collar 24 only by releasing the urgingforce by the drive means after filling of the compressed gas 22, therebyshortening man-hours of the gas filling work, and further simplifyingthe structure of the gas spring 7.

[0047] Incidentally, the structure of the gas filling jig 50 is notlimited to the above-described structure. The present invention can beapplied to any jig as long as the rod member 21 can be restricted at theupper limit position by the lock ring 26 and the annular collar 24 whenthe rod member 21 advances or retreats after filling of the compressedgas 22. Additionally, the present invention can be applied to a gasspring provided with a gas filling valve for filling compressed gaswithout using the gas filling jig 50. Moreover, the present inventioncan be applied to a gas spring for use in not only the tool fixingdevice 1 for the spindle 2 but also various machines or devices whichnecessitate strong urging functions.

1. In a gas spring comprising a cylinder main body; a rod memberslidably inserted into said cylinder main body; and a gas filled chamberinside of said cylinder main body and filled with compressed gas andapplying a gas pressure on said rod member so as to urge said rod memberin a first direction: said gas spring further comprises; a rod insertinghole for inserting said rod member which is formed on an end wall ofsaid cylinder main body in a second direction opposite to said firstdirection; an annular collar slidably fitted around said rod member at aposition apart from said rod inserting hole in said second direction; anannular groove formed on said annular collar, said annular groove beingopen toward said second direction; and a lock ring engaging with anannular ring mounting groove formed on said rod member; wherein saidlock ring is moved in said first direction together with said rodmember, to thus engage with said annular groove on said annular collar,so that said annular collar for stopping said rod member and said lockring is stopped by an end wall of said cylinder main body.
 2. A gasspring according to claim 1, wherein said lock ring is constituted suchthat its diameter can be reduced via one separated portion.
 3. In a gasfilling method for filling compressed gas into a gas spring comprising acylinder main body; a rod member slidably inserted into said cylindermain body; a gas filled chamber inside of said cylinder main body andfilled with compressed gas and applying a gas pressure on said rodmember so as to urge said rod member in a first direction; a rodinserting hole for inserting said rod member which is formed on an endwall of said cylinder main body in a second direction opposite to saidfirst direction; an annular collar slidably fitted around said rodmember at a position apart from said rod inserting hole in said seconddirection; an annular groove formed on said annular collar and beingopen toward said second direction; and a lock ring engaging with anannular ring mounting groove formed on said rod member: said gas fillingmethod is characterized by comprising steps of: filling said compressedgas into said gas filled chamber with the use of a gas filling jig;moving said rod member toward said second direction so as to allow saidlock ring to engage with said ring mounting groove after filling of saidcompressed gas; moving said rod member in said first direction by saidcompressed gas in said chamber in a state in which said lock ringengages with said ring mounting groove, so as to introduce said lockring into said annular groove formed on said annular collar; and lockingsaid annular collar on an end wall of said cylinder main body.
 4. A gasfilling method according to claim 3, wherein said gas filling jig isprovided with a case main body; a containing chamber in said case mainbody for containing said gas spring; and a gas filling hole formed insaid case main body for filling said compressed gas into said containingchamber; said case main body having; an outer cylindrical member to befitted around a portion on an end side in said first direction of saidrod member; a rod containing member containing a portion on an end sidein said second direction of the rod member; and an inner cylindricalmember extending from an end in said first direction of said rodcontaining member toward said first direction and containing thereinsaid cylinder main body.